1. Field of the Invention
The present invention relates generally to medium access control in wireless networks, and in particular to simultaneous half-duplex channel communications.
2. Description of Prior Art
Wireless Local Area Network (WLAN) protocols such as those based on the IEEE 802.11 standards are designed to recreate the high Quality of Service (QoS) that is typically supplied in wired networks that use standard LAN protocols such as Ethernet. High QoS includes uninterrupted network connections, high throughput and reliable delivery of data. However maintaining such high QoS in a WLAN is more difficult than in a wired network. The range and speed of wireless communications is often limited by, for example, interference and power limitations. Maintaining a high QoS in a WLAN therefore requires vigilant attention to error detection and correction and also requires careful monitoring of the conditions of the wireless link.
Despite their limitations, WLANs are frequently preferred over wired LANs, primarily because the user terminals of a WLAN are portable. Such portability makes possible Wireless Vehicle Area Networks (WVANs) and many Wireless Personal Area Networks (WPANs). However WLANs are also popular for other reasons. For example, with WLANs it is easy to use “ad hoc” networks that can be quickly assembled and torn down, and WLANs also may be more economical when compared with the high cost of infrastructure wiring.
The IEEE 802.11 standards concern the operation of a network's Media Access Control (MAC) layer. The MAC layer resides just above a network's Physical (PHY) layer and is responsible for controlling access to the wireless channel. The MAC receives MAC Service Data Units (MSDUs) from the higher layers. MSDU's may be fragmented into smaller MAC Protocol Data Units (MPDUs), which are then transported between network stations across the wireless medium. Network stations are devices connected to the network that may be mobile, portable, or stationary. MPDUs are transmitted between network stations using a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. Collision detection such as that used in the Ethernet protocol cannot be used in wireless transmissions, because when a wireless station is transmitting it cannot hear other stations on the network as its own signal will interfere with any received signal. If all stations, be it subscribers or APs, have equal priority and timing access to the channel via CSMA/CA, then the IEEE 802.11 standards refer to the above method of channel access as the Distributed Coordination Function (DCF).
The 802.11 standards also describe a second channel access method for networks. This method, referred to as the Point Coordination Function (PCF), requires an AP to be present, and it uses polling to provide access to the wireless medium. The AP constructs a polling list that determines the order in which the stations within the network will be polled.
In an IEEE 802.11 network, stations are collected into a Basic Service Set (BSS). A BSS may comprise an ad hoc network where all stations in the network can communicate directly with all other stations. Alternatively a BSS may include an AP in which case it is called an infrastructure BSS. In an infrastructure BSS, all stations communicate exclusively through the AP. The AP is often connected to a wired LAN and therefore can significantly increase the range and resources available to a BSS.
Extensions to the existing IEEE 802.11 protocol include the IEEE 802.11(e) QoS extensions. These are based on both the CSMA/CA channel access method and on the polling method. In an infrastructure BSS that is providing QoS, a QoS AP (QAP) must schedule all data downlinks to all stations in the BSS and all data uplinks from the stations to the QAP. To avoid delay and jitter, all uplinks and downlinks must be scheduled efficiently. Optimizing such scheduling using a scheduling algorithm is often a complex process that requires consideration of numerous variables such as the specific QoS requirements of individual stations, fading disruptions, processing time, variable queuing time, and the load of individual stations (i.e., the amount of data queued at a station waiting to be uplinked to the QAP).
Additional variables need to be considered when scheduling multicast data traffic. In a multicast environment, only one member of a many-to-many multicast group is able to operate as a data traffic source at any given time. Often such multicast groups involve half-duplex group voice communications requiring “push-to-talk-release-to-listen” switches. For example, emergency response teams such as police and firefighters may use half-duplex voice over IP (VoIP) communications equipment to multicast a dispatch call to all team members and then to receive a response from a single team member. These multicast communications work on top of Transmission Control Protocol/Internet Protocol (TCP/IP) based networks, and use multicast routers to transmit IP packets to multiple destinations.
In a half-duplex group voice communication network, at any given time, generally only one member of the group can be an active transmitter. Further, the active group member who has the authority to transmit must often change rapidly from one member to another. Identifying the active group member who has the authority to transmit, as part of a multicast scheduling process, is generally done by polling. Such polling includes the Request to Send/Clear to Send (RTS/CTS) procedures that are described in the 802.11 standard. However, the RTS/CTS procedures define a four-way handshake that results in higher overhead, because two extra packets are required to be transmitted for each payload. Further, such overhead is multiplied when intermediate stations must act as relay points, such as in many WVANs. Other solutions for transmission scheduling include Time-Division Multiplexing (TDM)/reservation techniques that are complex, and which are generally undesirable as they do not form part of the 802.11 standard. There is therefore a need for an improved method and system for minimizing polling overhead by reducing the required number of RTS/CTS packets.